The PNS is divided into somatic and autonomic divisions. If I'm right, it means: somatic nervous system ALWAYS receives the stimulus from outside of the organism and ALWAYS controls skeletal muscles; autonomic nervous system ALWAYS receives stimuli from inside of the organism, and its efferents ALWAYS end on smooth muscle, cardiac muscle or glands.

Is also part of the definitions above that somatic nervous system has a voluntary control over, autonomic (as its nam indicates) doesn't have voluntary control over it?

So in the patellar reflex, sensory neurons sensing strech in a muscle spindle of a skeletal muscle and motor neurons triggering contraction of a skeletal muscle are parts of the somatic nervous system (because the stimulus came from outside the body) but exceptionally they we don't have any voluntary control over them?

Then how come in some neurodegenerative disorders skeletal muscles contract involuntarily (tremor)? Did those motor neurons gain independence from voluntary control?

somatic vs autonomous is only a didactic classification. Here's a question to get you thinking: If you see a really hot girl and your heart starts beating faster, doesn't that contradict your definitions? Also, seeing food makes you secrete more saliva, doesn't it? but it is true that we have voluntary control over somatic processes and not over autonomic processes, because we define them like that.

Now, you must understand that in order to have voluntary control over something, the reflex controlling that something must close on the cortex - if it closes anywhere before that, you can't control it. For example, the patellar reflex is a monosinaptic reflex that closes in the spine. All your skeletal muscles are in a permanent state of mild contraction called muscular tonus, which is a reflex that closes in the cerebellum. In neurodegenerative disorders, what happens is that commands don't travel smoothly from the cortex to your muscles, things get screwed up along the way. So yeah, you are right in saying that motor neurons gain independence from voluntary control.

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

The hot girl example indeed supports what I just said. I can't control the wish so my heart receives sympathetic stimulation. This is a nice example to the activity of the autonomic nervous system. I'm more confused about involuntary skeletal muscle contraction during withdrawal reflex. That I would say is also an autonomic innervation (called reflex).

So is it possible that autonomic nervous system controls skeletal muscles, other than smooth- cardiac muscles and glands?

It's all clear what you said, just when it gets to written exams, you'd have to choose form A B C D, no chance to explain. Is it controlled by somatic or autonomic NS...